A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The demand for smaller and smaller semiconductor devices drives the need to have lithographic fabrication processes achieve pattern features and profiles having smaller critical dimensions. Moreover, such devices may comprise multiple layers, requiring precise positioning of successive layers over one or more prior layers. It is important that these smaller devices are consistently reproduced with as little overlay errors as possible to yield high-quality devices.
During a lithographic fabrication process, there are, however, numerous activities that contribute to overlay errors and compromise the quality of the exposed patterns. In particular, an exposed substrate is subject to thermal energy. In case of an optical lithographic apparatus the thermal energy substrate heating may result in deformations of a field on the substrate under exposure. In an immersion lithographic apparatus the substrate deformation is caused by immersion liquid evaporation that induces deformations of each field. Such thermally-induced deformations may result in unacceptable overlay and focusing errors and significantly reduce yield production.